1. Field of the Invention
The present invention relates to an injection molding machine, and more particularly to an injection unit of an injection molding machine using a ball screw/nut mechanism to perform an injection operation.
2. Description of Related Art
In most cases, an electrically-driven injection molding machine uses an injection unit that performs injection by driving a ball screw/nut mechanism by means of a motor to axially drive a pusher plate to thereby axially move an injection screw attached to the pusher plate in a rotatable but axially immovable manner.
FIGS. 9a and 9b are explanatory views of an injection unit that has heretofore generally been used. This injection unit includes a base 1 to which front and rear plates 2, 3 are fixed. A plurality of guide rods 4 are mounted between the front and rear plates 2, 3. A pusher plate 5 is arranged to move in the direction of injection (in the left and right direction in the drawing) while being guided by the guide rods 4. A ball screw 6 is fixed to the pusher plate 5 so as to be rotatable but axially immovable, and threadedly engaged with a ball nut 7 fixed through a load cell 12 to the rear platen 3 so as to be unrotatable and axially immovable.
An injection motor 8 is mounted to the pusher plate 5. A timing belt 11 is looped around a driving pulley 9 provided on the output shaft of the motor 8 and a driven pulley 10 attached to the ball screw 6. The ball screw 6 is rotatably driven by the motor 8.
An injection screw 13 is attached, in a rotatable but axially immovable manner, to a face of the pusher plate 5 on the side opposite a face thereof to which the ball screw 6 is mounted. A pulley 15 for rotating the injection screw 13 is attached to the injection screw 13 and adapted to be driven by a screw rotation motor, not shown. The injection screw 13 is inserted in a heating cylinder 14 fixed to the front plate 2.
Starting from a state shown in FIG. 9a, the injection screw 13 is rotated, whereby resin supplied into the heating cylinder 14 is kneaded and melted. At this time, a molten resin pressure is produced, which exerts to move the injection screw 13 backward against a back pressure given by the injection motor 8, etc., whereby the resin is kneaded and metered. FIG. 9b shows a state in the metering and kneading process where the injection screw 13, the pusher plate 5, and the ball screw 6 are moved backward. At the time of the backward movement, the ball screw 6 rotates while being moved backward in the axial direction. The motor 8 is driven to rotate the ball screw 6 through a transmission mechanism including the driving pulley 9, the timing belt 11, and the driven pulley 10, whereby injection operation is performed. Since the nut 7 threadedly engaged with the ball screw 6 is fixed to the rear platen in an unrotatable and axially immovable manner, the ball screw 6 moves axially when rotated, whereby the injection screw 13 is moved through the pusher plate 5 in the direction of injection (the axial direction of the injection screw 13), so that molten resin is injected into a die, not shown.
The above is to describe how the injection unit operates. When the injection screw 13, the pusher plate 5, and the ball screw 6 are moved backward as shown in FIG. 9b, the ball screw 6 tilts by gravity as indicated by θ in FIG. 9b that is only supported at its one end by the pusher plate 5, with another end unsupported. Thus, the ball screw 6 receives an offset load, and therefore, an offset load is applied also to the threaded coupling between the ball screw and the nut 7, resulting in a shortened life of the ball screw/nut mechanism.
The present invention contemplates to prevent an offset load from being applied to the ball screw. In relation to the present invention, an invention is known which comprises an auxiliary member coupled to a pusher plate by means of guide rods and in which a rear end of a ball screw rotatably supported by the auxiliary member is rotatably driven by a servomotor attached to the auxiliary member (See unexamined JP 2000-218663A and JP 2754356B).
Also known is the one in which a pusher plate arranged to be movable in the direction of injection is supported for linear motion by means of linear motion guides (see JP 2001-269974A).